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July 17, 2002
32
Processor Power Supplies and Power
Management
Example PLL Power Supply Circuit Description
R1/Q2 and R2 form a voltage divider to create a 1.0 V reference for the minimum V_CPU_PLL
operating voltage. A high on START_SEQ (typically connected to the POWERGOOD output of the
V_CPU_CORE regulator) turns on the PLL supply via FET Q2. U1A and Q1 are connected as a
unity-gain voltage follower so that V_CPU_PLL will equal the positive input of the opamp U1A at pin
3. U1A pin 3 is then connected to V_CPU_CORE through resistor R3. The purpose of R3 is to
separate the U1A input node from V_CPU_CORE. The circuit described so far is an accurate
voltage follower, where V_CPU_PLL tracks V_CPU_CORE. U1B is also connected as a voltage
follower for the 1.0 V reference circuit, and D6 allows U1B to take control of the loop if V_CPU_PLL
goes below 1.0 V. R4 provides a load for the U1A-Q1 follower. The PLL power supply circuit should
be located as close to the processor V_CPU_PLL pin as possible, and the V_CPU_PLL/GND loop
length should be minimized.
Circuit Operation
During operation, the V5_0 (5.0 V) supply to the opamp and V2_5 (2.5 V) supply to Q1 must be on
whenever the processor core voltage (V_CPU_CORE) is on. Normal circuit operation is as follows:
Operation above 1.0 V
: U1A regulates the V_CPU_PLL output to the voltage on its positive input
(pin 3). If V_CPU_CORE is higher than 1.0 V, then V_CPU_PLL follows it. This makes the negative
input of U1B (pin 6) greater than the 1.0 V reference on its positive input (pin 5), driving its output
(pin 7) low. When pin 7 goes low it back-biases D6, disconnecting the U1B output from the U1A
positive input and allowing V_CPU_PLL to track V_CPU_CORE.
Operation below 1.0 V
: When V_CPU_CORE goes below 1.0 V, V_CPU_PLL attempts to track it,
causing the negative input (pin 6) of U1B to fall below the 1.0 V reference on the U1B positive input
(pin 5). This in turn causes the U1B output (pin 7) to increase, which forward biases D6 and allows it
to take control of the loop. U1B now regulates V_CPU_PLL to its 1.0 V reference value as long as
V_CPU_CORE is below the 1.0 V reference.
Содержание Crusoe TM5500
Страница 1: ...TM5500 TM5800 System Design Guide July 17 2002...
Страница 6: ...July 17 2002 6 List of Tables...
Страница 8: ...July 17 2002 8 List of Figures...
Страница 50: ...July 17 2002 50 Processor Power Supplies and Power Management...
Страница 110: ...July 17 2002 110 System Design Considerations...
Страница 122: ...July 17 2002 122 System Design Checklists...
Страница 128: ...July 17 2002 128 Serial Write protection PLD Data...
Страница 130: ...July 17 2002 130 Index...